4 - Central Institute of Brackishwater Aquaculture

National Workshop-cum-Training on Biolnformatla and Information Management in Aquaculture
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Figure 2. Electromagnetic spectrum showing regions used for remote sensing
A microwave imaging system which can produce high-resolution image of the
Earth is the synthetic aperture radar (SAR). The intensity in a SAR image
depends on the amount of microwave backscattered by the target and received
by the SAR antenna. Since the physical mechanisms responsible for this
backscatter is different for microwave, compared to visible/infrared radiation, the
interpretation of SAR images requires the knowledge of how microwaves interact
with the targets.
In satellite remote sensing of the earth, the sensors are looking through a layer
of atmosphere separating the sensors from the Earth's surface being observed.
Hence, it is essential to understand the effects of atmosphere on the
electromagnetic radiation traveling from the Earth to the sensor through the
atmosphere. The atmospheric constituents cause wavelength dependent
absorption and scattering of radiation. These effects degrade the quality of
images. Some of the atmospheric effects can be corrected before the images are
subjected to further analysis and interpretation.
A consequence of atmospheric absorption is that certain wavelength bands in the
electromagnetic spectrum are strongly absorbed and effectively blocked by the
atmosphere. The wavelength regions in the electromagnetic spectrum usable for
remote sensing are determined by their ability to penetrate atmosphere, which
depends on its transmittance properties. These regions are known as the
atmospheric windows. Remote sensing systems are often designed to operate
within one or more of the atmospheric windows in optical and microwave regions
of electromagnetic spectrum. These windows exist in the microwave region,
some wavelength bands in the infrared, the entire visible region and part of the
near ultraviolet regions (Figure 3). Although the atmosphere is practically
transparent to x-rays and gamma rays, these radiations are not normally used in
remote sensing of the earth.
When solar radiation hits a target surface, it may be transmitted, absorbed or
reflected. Many remote sensing systems operate in the wavelength region in
which the reflected energy predominates. The reflectance properties of the earth
surface features may be quantified by measuring the fraction of incident energy